Traditionally, detergent powders were produced by spray drying. However, the spray drying process is both capital and energy intensive and the products were quite bulky, having a relatively low bulk density.
The desire for powders with higher bulk densities led to the development of processes which employ mainly mixing, without the use of spray drying. These mixing techniques offer great flexibility in producing powders of various different compositions from a single plant, by post-dosing various components after an initial granulation stage. The resultant powders have fairly high bulk densities, which is desirable for some product forms. However, many of these non-spray drying techniques are unsuitable for production of powders over a wide bulk density range and in particular, for the production of lower bulk density powders.
One kind of process, which does not involve spray-drying, and which is capable of producing medium bulk densities between those of spray dried and other non-spray dried powders, involves use of a low shear granulator, usually a fluidised bed apparatus. Although fluidised bed granulation processes per se can give good control of bulk density, there can still be problems with poor dispensing of the resultant product.
EP-A-360 330 discloses a process in which a spray dried base powder is sprayed in the low shear mixer with a liquid binder comprising a fatty acid and a nonionic surfactant to improve dispensing properties of the final product.
However, we found that for granulated powders such a coating does not reliably provide adequate dispensing behaviour.
For overall powder quality it is important to find the right balance between fast dispensing and fast enough dissolution. The techniques used to improve dispensing should not deteriorate solubility beyond unacceptable limits. At the same time, the dispensing improvement of the powder should be robust, e.g. during handling and transportation of the product, where particle breakdown may occur.
We have now found that dispensing properties of non-spray dried materials are significantly improved in a granulation method in which solids are granulated in a two-stage process in which the second stage is effected in a low shear granulator such as of fluid bed type and a liquid binder is applied in both stages.